What are idempotency and the outbox pattern?

Understand the concept

Idempotency makes repeated requests for one logical operation produce one intended effect. The outbox pattern stores an event record in the same database transaction as the business change, allowing a later publisher to send it reliably. These solve related but different failure windows: duplicate work and losing the message between a commit and publication.

Important ideas

• Idempotency makes repeated processing have one intended effect; enforce keys atomically.

• Outbox: save business changes and an event record in one database transaction.

• A publisher sends stored events later; consumers still need duplicate handling.

• Store a unique operation key and result atomically, and reject reuse of a key for a different request payload.

Outbox flow:

1. Start a database transaction.
2. Save the business change and an outbox row with a stable event ID.
3. Commit both together.
4. A publisher reads unsent outbox rows and sends the events.
5. Mark publication progress; a crash before this step can cause redelivery.
6. A consumer uses the event ID or an idempotent operation to avoid duplicate effects.